The present invention relates to an apparatus and to a process making it possible to measure wave surface distortions introduced by an optical system and particularly by an objective or lens. These distortions are mainly linked with the transfer function of the optical system in coherent illumination. This measurement is performed on the basis of the study of the transmission, by the optical system, of coherent and sine light waves.
The principles of such a measurement will now briefly be described. In coherent light, the image of a sinusoidal intensity distribution, supplied by an optical system, is a sinusoidal intensity distribution of the same amplitude, but of different phase. The phase of the image sinusoidal distribution is dependent on the transmission quality of the optical system. This phase .phi. (.nu.) which defines the transfer function of the optical system in coherent illumination is dependent on a coefficient .DELTA.(.nu.), which characterizes the wave surface distortions introduced by the optical system and which, itself, depends on the spatial frequency .nu. of the light wave.
The definition of the coefficient .DELTA.(.nu.) is illustrated in FIG. 1, in which .SIGMA..sub.o and .SIGMA. effectively represent the reference wave surface and the real wave surface of the light wave transmitted by the optical system. In the case of a perfect optical system, i.e. without aberrations, the wave surface .SIGMA. and the wave surface .SIGMA..sub.o would coincide. The reference wave surface .SIGMA..sub.o can be defined as the sphere centered on image point A and tangential to the exit pupil P of the optical system.
The straight line d forms an angle .theta. with axis x representing the propagation direction of the light wave. This line d intersects the reference wave surface .SIGMA..sub.o at a point I and the real wave surface .SIGMA. at a point J. The algebraic value IJ represents the coefficient .DELTA.(.nu.).
For a given light wave length .lambda., the spatial frequency .nu. of the wave transmitted by the optical system is defined by the relation .nu.=(sin .theta.)/.lambda. and the phase .phi.(.nu.) of this wave by ##EQU1##
The known apparatuses do not permit direct determination of the coefficient .DELTA.(.nu.) of the optical system and consequently directly determine the wave surface distortions introduced by it. The determination of these distortions by means of the prior art apparatus is very difficult to perform.